The present invention relates to a process for the purification of uranium hexafluoride containing traces of plutonium fluoride and/or neptunium fluoride.
It is known that one of the main sources of uranium hexafluoride for the separation of uranium isotopes by gaseous diffusion is the reprocessing of irradiated fuels. During the reprocessing, the fuels are generally treated by a wet process to separate the uranium, plutonium and other transuranium elements. At the end of this reprocessing, the uranium is generally recovered in the form of a uranium nitrate solution, which still contains traces of neptunium and plutonium producing alpha radiation of approximately 5000 d.p.m. (disintegrations per minute) per gram of uranium (UNIREP standard &lt;1500 dpm g.sup.-1 U with for Pu/U.sub.total &lt;10.sup.-8). This uranyl nitrate is then converted by fluorination into uranium hexafluoride, which therefore also contains traces of neptunium and plutonium in hexafluoride form. However, after the fluorination phase, the neptunium and plutonium activity is reduced. Nevertheless, it is desirable to reduce to the lowest possible value, the level of neptunium hexafluoride and plutonium hexafluoride in the uranium hexafluoride in order to introduce the minimum of .alpha. radioactive emitters, other than uranium isotopes, into isotopic separation plants. Moreover, the adoption of this process would make it possible to considerably reduce the presently accepted standard of 1500 .alpha. disintegrations, other than those of the uranium isotopes, per minute and per gram of uranium in UF.sub.6 form.
The process hitherto used for purifying gaseous uranium hexafluoride containing plutonium hexafluoride or neptunium hexafluoride as impurities has consisted of fixing the volatile fluorides to a solid fluoride, such as sodium fluoride or magnesium fluoride, and then vaporizing in a preferred manner the uranium hexafluoride by heating the solid complex. Consideration has also been given to the use of other metal fluorides, such as cobalt fluoride and sulphur tetrafluoride, in order to selectively reduce the neptunium hexafluoride and plutonium hexafluoride and in this way to separate them from the uranium hexafluoride. Such processes are more particularly described in French Pat. Nos. 2,111,730, and 2,034,805, as well as in U.S. Pat. Nos. 3,046,089, 3,708,568 and 3,625,267.
However, such processes do not make it possible to purify under good conditions uranium hexafluoride which also contains traces of F.sub.2 or ClF.sub.3, because these highly fluorinating products firstly react on the cobalt fluoride CoF.sub.2 in order to give CoF.sub.3, which is a solid. This leads to the formation of a relatively impermeable crust, which is prejudicial to the reaction of the remaining CoF.sub.2 with PuF.sub.6 and NpF.sub.6.